SPI: SPI.end() before SPI.begin() causes the board to become unresponsive


### Issue:
Calling `SPI.end()` before `SPI.begin()` causes the board to become unresponsive.

### Example:

```cpp
#include <SPI.h>

void setup() {
  SPI.end();
}

void loop() {
}
```

### Expected behavior

 `SPI.end()` should return safely if SPI has not been initialized.

### Hardware

* Arduino UNO R4 Minima

### Core version

* ArduinoCore-renesas 1.6.0

A fix is proposed in #551 .
